As used herein, the term solid/liquid probe refers to a shaft and flange assembly that allows introduction of a sample on the surface of a heat resistant material into a heated gas stream in the atmospheric pressure ion (API) source of a mass spectrometer. The term ESI probe refers to a commercially available device for ionization of analyte in a liquid stream using a high voltage that is interfaced to a mass spectrometer through the API source. The term APCI probe refers to a commercially available device for ionization of analyte in a liquid by nebulizing the liquid into droplets and vaporizing the liquid droplets with subsequent ionization using a corona discharge with mass analysis by a mass spectrometer. The current practice in mass spectrometry is to have either APCI or ESI ionization methods, both of which ionize analyte from a liquid stream. No commercial API instrument includes a direct solids/liquid introduction probe.
Atmospheric pressure ionization mass spectrometers (APIMS) instruments currently available lack flexibility. They primarily accept only liquid effluent from which analyte ions are produced by electrospray ionization, atmospheric pressure chemical ionization, or photoionization. A recent configuration has been published in which a gas chromatograph was also interfaced to the API source so that either a liquid or a gas stream from a gas chromatograph could be ionized (WO 2006/060130 A2, McEwen). Typically, primary ions are formed at atmospheric pressure by initiation of a gaseous electrical discharge by an electric field or by electrospray ionization (ESI) as described in U.S. Pat. No. 6,297,499 (Fenn) and; U.S. Pat. No. 5,788,166 (Valaskovic). The primary ions in turn ionize the gas phase analyte molecules by either an ion-molecule process as occurs in atmospheric pressure chemical ionization (APCI), by a charge transfer process, or by entraining the analyte molecules in a charged droplet of solvent produced in the electrospray process. In the case of analyte being entrained in a charged liquid droplet, the ionization process is the same as in electrospray ionization (ESI) because the analyte molecules are first entrained in the liquid droplets and subsequently ionized.
Electrospray ionization (ESI) is a powerful method for producing gas phase ions from compounds in solution. In ESI, a liquid is typically forced from a small diameter tube at atmospheric pressure. A spray of fine droplets is generated when a potential of several thousand volts is applied between the liquid emerging from the tube and a nearby electrode. Charges on the liquid surface cause instability so that droplets break from jets extending from the emerging liquid surface. Evaporation of the droplets, typically using a counter-current gas, leads to a state where the surface charge again becomes sufficiently high (near the Raleigh limit) to cause instability and further smaller droplets are formed. This process proceeds until free ions are generated by either the evaporation process described above or by field emission that occurs when the field strength in the small droplets is sufficiently high for field evaporation of ions to occur. Molecules more basic than the solvent being used in the ESI process are preferentially ionized. Because ESI generates gas phase ions from a liquid, it is an ideal ionization method for interfacing liquid chromatography (LC) to mass spectrometry (MS). The power of ESI for the analysis of compounds as large and diverse as proteins won the 2003 Nobel prize in Chemistry for John Fenn. The combination of ESI with MS with liquid separation methods is extremely powerful analytically and results in large numbers of LC/MS instruments being sold each year.
Because ESI is most sensitive and most suitable for basic and polar compounds, most LC/MS instrumentation incorporates an alternative atmospheric pressure ionization technique called atmospheric pressure chemical ionization (APCI). APCI was initially developed by Horning, et al. using 63Ni beta decay for ionization. See Horning, E. C.; Horning, M. G.; Carroll, D. I.; Dzidic, I.; Stillwell, R. N., New Picogram Detection System Based on a Mass Spectrometer with an External Ionization Source at Atmospheric Pressure. Anal. Chem., 1973. 45: p. 936-943. A discharge ion source has since replaced the 63Ni as the source of ionization. A discharge is generated when a voltage, typically applied to a metal needle, is increased to a range where electrical breakdown (formation of free electrons and ions) of the surrounding gas occurs (typically several thousand volts). The primary use of this ionization method has been as an ionization interface between liquid chromatography and mass spectrometry. See Dzidic, I.; Carroll, D. I.; Stillwell, R. N.; Horning, E. C., Comparison of Positive Ions formed in Nickel-63 and Corona Discharge Ion Sources using Nitrogen, Argon, Isobutene, Ammonia and Nitric Oxide as Reagents in Atmospheric Pressure Ionization Mass Spectrometry. Anal. Chem., 1976. 48: p. 1763-1768. This ionization method relies on evaporation of the liquid exiting the liquid chromatograph with subsequent gas phase ionization in a corona discharge. The primary ions produced in the corona discharge are from the most abundant species, typically nitrogen and oxygen from air or solvent molecules. Regardless of the initial population of ions produced in the corona discharge, diffusion controlled ion-molecule reactions will result in a large steady state population of protonated solvent ions. These ions in turn will ionize analyte molecules by proton transfer if the reaction is exothermic or by ion addition if the ion-molecule product is stable and infrequently by charge transfer reactions. While this technique tends to be more sensitive than ESI for low molecular weight and less polar compounds, it nevertheless is not sensitive for highly volatile compounds and those less basic than the LC solvent. Thus, neither APCI nor ESI are good ionization methods for a large class of volatile and less polar compounds. For this reason, other ionization methods, such as photoionization have been applied to LC/MS to more effectively reach a subset of this class of compounds (See, for example U.S. Pat. Nos. 7,002,146, 5,245,192, 6,646,256, 6,630,664, US20030111598). Photoionization at atmospheric pressure uses an ultraviolet (UV) source for ionization of gas phase molecules. Typically, a plasma-induced discharge lamp that produces radiation in the range of 100-355 nm is used to generate ionization. Such a source is sold by Synagen Corporation for use with LC/MS.
Thus, liquid chromatographs interfaced with the atmospheric pressure ionization methods of ESI and APCI are in common use and frequently the mass spectrometers associated with these ionization methods have advanced analytical capabilities such as MSn (MS/MS, MS/MS/MS, etc.) and/or high mass resolution and accurate mass analysis. However, LC/MS instruments do not effectively address a large class of important volatile and less polar compounds. Herein is described atmospheric pressure ionization of vaporizable compounds introduced into the ionization region as a neat liquid, solid, or as tissue or materials on instruments designed for LC/MS applications without interference with the operation of these liquid introduction techniques.
Solid probe introduction is commonly interfaced to mass spectrometers which use vacuum ionization methods such as electron or chemical ionization. The solid probe is limited to molecules that can be made to vaporize in vacuum by application of heat. Because solid probes on current mass spectrometers interface with ion sources that operate in vacuum, it must be inserted into the mass spectrometry through a pressure drop device. Commonly, the pressure drop device is a ball valve device with polymeric “O”-rings that seal the probe so that a vacuum can be achieved through a roughing pump before the ball valve is opened. Because this is a time intensive process and involves inserting the sample into vacuum, volatile compounds can be pumped away. Further, the device is available only on instruments having chemical and/or electron ionization, methods that operate substantially below atmospheric pressure.
To our knowledge, the only references to use of a direct insertion probe for introduction of samples into an API source are articles by McEwen, et al. (See McEwen, C. N.; McKay, R. G.; Larsen, B. S., Analysis of solids, liquids, and biological tissues using solids probe introduction at atmospheric pressure on commercial LC/MS instruments, Anal. Chem., 2005, 77, 7826-7831. McEwen, C.: Gutteridge, S., Analysis of the Inhibition of the Ergosterol Pathway in Fungi using atmospheric solids analysis probe (ASAP) method, J. Am. Soc. Mass Spectrom., 2007, 18, 1274-1278.) In these articles the only description given of the solids/liquid introduction was use of a Teflon® plug in a glass sleeve with a hole drilled through for insertion of a melting point tube into a hot nitrogen stream within the API source housing. A major disadvantage of this method is that in order for the melting point tube to align with the hot nitrogen the hole through which the tube was inserted had to be a tight fit. Inserting the melting point tube through the opening with sample on the exterior of the melting point tube resulted in sample being deposited on the Teflon® plug with subsequent cross contamination of later runs. Another drawback of the arrangement was the melting point tube, being made of glass, was hand held and would sometimes break when inserted through the plug which could result in injury. Finally, the probe was describe only for a QT of ‘fishbowl’ ion source housing which required drilling a hole through a glass sleeve which was a difficult process even for glass blowing experts. This approach is not viable on commercial instruments nor is it practical from a manufacturing point of view. Further, no description has been given of such a probe which does not interfere with the normal ESI/APCI operation of the mass spectrometer. In addition, no mention was made of using a fine stream of heated gas to image chemical composition from a surface nor was mention made of using adsorption materials for sample concentration.
Dzidic, et al. described the use of platinum wire to introduce chloro-nitrobenzene by volatilization into a specially built API source that used 63Ni as the source of ionization. The only description was that the platinum wire was resistively heated in a stream of nitrogen gas. From ion source descriptions in other publications, it is likely that the sample was introduced into the nitrogen stream outside the ion source and carried into the ionization region through a heated tube similar to the GC/API-MS experiments these authors carried out. (See Dzidic, I.; Carroll, D. I.; Stillwell, R. N., Horning, E. C., Atmospheric Pressure Ionization (API) Mass Spectrometry: Formation of Phenoxide Ions from Chlorinated Aromatic Compounds, Anal. Chem., 1975, 47, 1308-1312.). An open source experiment was reported in which a sample on a surface was ionized using charged droplets either from an electrospray device or from APCI in which the droplets were charged using a corona discharge. (See Cotte-Rodriquez, I.; Takats, Z.; Talaty, N.; Chen, H.; Cooks, R. G., Desorption Electrospray Ionization of Explosives on Surfaces: Sensitivity and Selectivity Enhancement by Reactive Desorption Electrospray Ionization, Anal. Chem., 2005, 77, 6755-6764.) The mechanism for this device is believed to be charge droplets hitting a surface with subsequent pickup of sample into smaller droplets that spatter into the gas phase with subsequent ionization. Another open source ionization method for direct analysis in real time uses an electric discharge device to produce metastable nitrogen or helium species which form reactant ions that when directed at analyte produces analyte ions (U.S. Pat. No. 7,112,785 B2, Laramee and Cody). Neither technique describes the use of heated gas to vaporize materials and both devices use open air sources which have the potential to emit hazardous gases into the surrounding area. Several patents describe multi-probe sources, non of which use a direct introduction solid/liquid introduction probe (US20010013579 A1, Andrien, Whitehouse, Shen, Sansone, U.S. Pat. No. 7,078,681 B2, Fischer, Gourlen, Bertsch, US20060255261 A1, Whitehouse, White, Willoughby, Sheehan).
Because work in the Horning group in the 1970's developed the APCI technique, we give here some important references. A review paper by E. C. Horning, et al discusses both GC/APIMS and LC/APIMS ion sources (See Horning, E. C.; Carroll, D. I.; Dzidic, I.; Haegele, K. D.; Lin, S.-N.; Oertil, C. U.; Stillwell, R. N., Development and Use of Analytical Systems Based on Mass Spectrometry. Clin. Chem., 1977. 23(1): p. 13-21). This article shows diagrams of each ion source and refers back to two previous publications for details on LC/APIMS and on GC/APIMS. (Respectively see Carroll, D. I.; Dzidic, I.; Stillwell, R. N.; Haegele, K. D.; Horning, E. C., Atmospheric Pressure Ionization Mass Spectrometry: Corona discharge Ion Source for use in a Liquid Chromatography-Mass Spectrometry-Computer Analytical System. Anal. Chem., 1975. 47: p. 2369-2373 and see Dzidic, I.; Carroll, D. I.; Stillwell, R. N.; Horning, E. C., Comparison of Positive Ions formed in Nickel-63 and Corona Discharge Ion Sources using Nitrogen, Argon, Isobutene, Ammonia and Nitric Oxide as Reagents in Atmospheric Pressure Ionization Mass Spectrometry. Anal. Chem., 1976. 48: p. 1763-1768.
Commercial mass spectrometers have been manufactured that analyze gaseous compounds using corona discharge APCI, e.g. ABB, Inc., Extrel Quadrupole mass spectrometers, described in Ketkar, S. N.; Penn, S. M.; Fite, W. I., Real-time Detection of Parts per Trillion of Chemical Warfare Agents in Ambient Air Using Atmospheric Pressure Ionization Tandem Quadrupole Mass Spectrometry. Anal. Chem., 1991. 63: p. 457-459. and Sciex. mass spectrometers, described in Lave, D. A.; Thompson, A. M.; Lovett, A. M.; Reid, N. M., Adv. Mass Spectrom., 1980. 8B: p. 1480. and Reid, N. M.; Buckley, J. A.; Pom, C. C.; French, J. B., Adv. Mass Spectrom., 1980. 8B: p. 1843. Two patents (EP 0819937 A2 and U.S. Pat. No. 5,869,344) which disclose use of a Venture pump in combination with water vapor introduction for analysis of trace volatiles in air from sources such as breath and fragrances emulating from skin and clothing. Papers by L. Charles, et al and by G. Zehentbauer, et al have been published that reportedly improve on this method. (Respectively see Charles, L.; Riter, L. S.; Cooks, R. G., Direct Analysis of Semivolatiel Organic Compounds in Air by Atmospheric Pressure Chemical ionization Mass Spectrometry. J. Agric. Food Chem., 2000. 48: p. 5389-5395. and see Zehentbauer, G.; Kirck, T.; Teineccius, G. A., J. Agric. Food Chem., 2000. 48: p. 5389-5395.) All of these methods introduce the sample into the API ionization region as a vapor and not directly as described in this application.
Pyrolysis with ionization of the gaseous pyrolysate has been reported, (see Snyder, A. P.; Kremer, J. H.; Mouzelaar, H. L. C.; Windig, W.; Taghizahed, K., Curie-point pyrolysis atmospheric pressure chemical ionization mass spectrometry: preliminary performance data for three biopolymers. Anal. Chem., 1987. 59: p. 1945-1951. while W. E. Steiner, et al has reported APCI of warfare agent simulants (see Steiner, W. E.; Glowers, B. H.; Haigh, P. E.; Hill, H. H., Secondary Ionization of Chemical Warfare Agent Simulants: Atmospheric Pressure Ion Mobility Time-of-Flight Mass Spectrometry. Anal. Chem., 2003. 75: p. 6068-6076.
A wafer thermal desorption system has been described for introducing samples into APIMS (in published US patent application US2002148974). Several patents (for example, JP2002228636, WO2002060565, U.S. Pat. No. 6,474,136, US2003092193, US2003086826, U.S. Pat. No. 6,032,513, U.S. Pat. No. 6,418,781, JP09015207, and JP06034616) discuss the use of GC and APIMS for the analysis and quantitation of trace gases such as hydrogen, oxygen, argon, carbon dioxide, carbon monoxide, freons, silanes, and other compounds that are gases at ambient temperature, primarily for the semiconductor industry. McLuckey, et al. (Atmospheric Sampling Glow Discharge Ionization Source for the Determination of Trace Organic Compounds in Ambient Air, Anal. Chem., 60, 1988, 2220-2227.) disclosed a method for observing volatile organic compounds in air using a glow discharge.
Currently available mass spectrometers do not combine LC/MS and solids/liquid probe in a single instrument or use API solids/liquid probes in any fashion. The great majority of mass spectrometers are either designed for LC/MS operation or vacuum ionization operation with a solids probe, but not both. Many laboratories will have both instruments with solids probe with a vacuum ionization source and LC/MS instruments with API ionization available, but a growing number of laboratories have only API LC/MS instrumentation. Therefore, it is desirable to devise an ionization source that allows commonly available LC/MS mass spectrometers to also be capable of direct sample introduction by means of a direct introduction liquid/solids probe. Such an instrument would extend the coverage of compounds that can be analyzed by currently available LC/MS instruments. Such an interface probe would have the additional advantage that the advanced capabilities common in LC/MS instruments, but not common in vacuum ionization instruments (e.g. techniques known to those practiced in the art such as cone-voltage fragmentation, MSn, high-mass resolution, accurate mass measurement) would become available to liquids/solids direct analysis without purchase of new and expensive instrumentation.
Further, the heated gas stream used to vaporize materials reduces thermal fragmentation relative to direct resistive heating of samples off of a metal wire or ribbon. Apparently, the gas stream applies heat to the sample/air interface so that molecules are immediately removed from the surface when they attain sufficient thermal energy to overcome surface forces. Application of a thin stream of hot gas allows selected areas of a surface to be heated with vaporization of volatile and semi-volatile compounds. Imaging of the surface for these compounds which may be metabolites, for example, becomes available. The compounds vaporizing from the surface are ionized by a corona discharge or by photoionization. Alternatively, a resistive heater or conductive heating can be used to vaporize compounds, the resistive heating method being especially applicable for pyrolysis.